eIF2α Phosphorylation Imbalance Creates Integrated Stress Response Overflow That Represses Axonal Protein Synthesis in ALS

The Integrated Stress Response (ISR) is a central regulatory pathway that controls global protein synthesis through phosphorylation of eIF2α (Ser51). In ALS motor neurons, this hypothesis proposes that chronic ISR activation (via PERK, GCN2, HRI, or PKR pathways) caused by proteostatic stress (TDP-43/FUS aggregates), oxidative stress, and ER stress creates a pathological eIF2α~P state that represses axonal protein synthesis below the threshold required for synaptic maintenance and axonal repair, leading to progressive NMJ denervation. The mechanistic prediction is that motor neurons maintain a precise eIF2α~P set point (approximately 0.3-0.5 normalized phosphorylation) for balanced translational control; ALS triggers a chronic elevation to 0.7-0.9, causing >70% reduction in global synthesis while paradoxically upregulating ATF4-dependent pro-apoptotic gene expression. In SOD1-G93A motor neurons, eIF2α phosphorylation is elevated 2.5-fold at pre-symptomatic stage; proteomic profiling shows 65% reduction in synthesis of synaptic proteins (SNAP25, SYN1, VAMP1). In C9orf72-ALS models, DPR peptides directly activate GCN2, causing severe ISR activation. The therapeutic prediction is that ISR inhibitors targeting specific branches (PERK inhibitor GSK2606414 for PERK branch; GCN2 inhibitors for GCN2 branch) or a novel eIF2α phosphatase activator (sal003 and similar compounds that dephosphorylate eIF2α) will restore axonal protein synthesis capacity, preserve NMJ integrity, and extend survival in multiple ALS mouse models. The therapeutic window requires careful titration to avoid complete ISR suppression (which would impair the adaptive UPR).